The present invention relates to an implantable energy conversion system utilizing skeletal muscle power to operate an implanted device and, in particular, to a system for converting the linear contractile force of a muscle into sufficient power for operating of a circulatory support device.
Due to the dramatic increase in successful heart transplantations in recent years, today's heart disease patients more readily elect to undergo this procedure and generally expect that a donor heart will be available when required. Yet, the data shows that 88% to possibly as high as 96% of those that need a donor heart will not receive one due to the very limited number available (2,000 per year against a need of 17,000 to 50,000 per year in the United States alone). Those patients awaiting heart transplantation demonstrate the clear but unfulfilled need for a chronic artificial heart device as an alternative to heart transplantation. A system is needed to provide a therapeutic alternative to cardiac transplantation which, because of the limited supply of donor hearts, can only meet the needs of a fraction of the patients who could benefit annually from cardiac replacement or assistance. Thus, some form of chronic circulatory support is needed if large groups of patients with Congestive Heart Failure ("CHF") are to have the opportunity to live out a near-normal life span with a reasonable quality of life.
Attempts have been made to meet this need by developing externally powered mechanical artificial hearts. In December 1981, a Symbion artificial heart was implanted in Barney Clark in an orthotopic position with the natural heart excised. That same year, Thoratec Laboratories Corporation was preparing the Pierce-Donachy design Ventricular Assist Device (VAD) for clinical trials as a VAD in the left ventricle, right ventricle, or both positions, thus serving as a heterotopic artificial heart with the natural heart left in place. In April 1982, the first clinical ease with the Thoratec.TM. VAD System was implanted. The clinical work over the past eight years has shown that patients do quite well when adequate circulation of the blood is restored and maintained before the onset of irreversible cardiogenic shock.
Pneumatic systems require that the patient be tethered to a bulky, external console which powers and controls the pneumatic system. The National Heart Lung and Blood Institute (NHLBI) has been supporting the development of electrically powered totally implantable artificial hearts and left ventricular assist systems and clinical trials are soon to begin with one such system. At present, the major limitation of the clinical work with circulatory support devices is the need for simple, reliable implantable power sources for driving the blood pumps, and thus allowing the patients to be discharged from the hospital in order to resume a near normal life. Electromechanical systems have the disadvantage of requiring bulky hardware for external power, transcutaneous electrical power transmission systems, implantable batteries and other components.
The use of skeletal muscle power provides a new opportunity for realizing completely implantable tether-free mechanical circulatory support, free from any external power sources such as batteries. Skeletal muscle powered circulatory support systems have the potential of providing a simpler alternative to electromechanical systems, and of offering an improved quality of life for the patient by eliminating the need for electricity (except for the low power requirements of muscle stimulation). All of the associated external and implanted power conditioning hardware, batteries, coils, and the like could be eliminated and replaced by natural muscle biochemical and biophysical processes.
The key problem with skeletal muscle as a power source is how to harness the available energy and utilize it efficiently for maximal circulatory support. Experimental work focused primarily on the physiology of skeletal muscle while it was being conditioned (or transformed) from predominantly fast-twitch muscle fibers, susceptible to fatigue into muscle bundles with predominately slow-twitch muscle fibers capable of chronic periodic contractions. As it was established that such transformed skeletal muscle was capable of long-term stimulation-contraction, surgeons began clinical applications. In one application, called dynamic cardiomyoplasty, the distal end of the latissimus dorsi muscle was carefully dissected from its natural anatomical position across the lateral posterior area of the back and moved into the thoracic cavity, where it was then wrapped around the heart. After a three to four week healing period, stimulation of the transformed latissimus dorsi was initiated. As it contracted, the coiled muscle shortened, thus causing it to squeeze the epicardial surfaces of the heart. When timed properly in synchrony with the heart, this action was meant to cause an augmentation of the pumping function of the heart. Clinical experience to date on dynamic cardiomyoplasty, where the latissimus dorsi is wrapped around the heart, has demonstrated (with patient follow-up out to several years) that a chronically stimulated fatigue-resistant skeletal muscle as a long term power source is quite feasible. However, there is considerable controversy over the actual amount of assistance and patient benefit provided with this technique.
To obtain more cardiac assistance other investigators have proposed using muscles such as the latissimus dorsi or rectus abdominis to form a blood pump independent and distinct from the heart. In 1960, Kantrowitz demonstrated diastolic counterpulsation obtained with a diaphragm muscle wrapped around the distal thoracic aorta (Kantrowitz, Trans. ASAIO, 6:305, 1960). In 1964, Kusserow demonstrated actuation of a pump by direct linear contraction of a muscle. The quadriceps muscle of a dog was mobilized and attached to the handle of an external pump; the pump could be operated for up to 8 hours by electric stimulation of the muscle. (Kusserow, et al. (1964) Trans. ASAIO, 10:74). However, methods for internalizing a pump relying on linear contraction of a muscle have not been developed in the art. Rather, developments have been based on compression by transformed skeletal muscle. These developments include aortic counterpulsation devices with a muscle wrapped around a pouch connected to the aorta (Acker, et al., 1987, J. Thoracic Cardiovasc. Surg., 94:163-174, U.S. Pat. No. 5,007,927), and a dual-chamber system with the muscle wrapped around one chamber coupled to a blood pump chamber used for counterpulsation (U.S. Pat. No. 4,979,936). Apical-to-aortic valved conduits, similar in placement to today's ventricular assist devices, have also been fabricated with muscles wrapped around the pumping chamber (U.S. Pat. Nos. 4,813,952 and 4,759,760). Most of these approaches have demonstrated inadequate power available for left ventricular assistance and as a result, a number of groups have suggested that skeletal muscle ventricles might be more appropriate for right heart assist (Bridges, et al., 1989, Circulation, 80 (supp. III): III 183-III 191) Sakakibara, et al., 1990, Trans. ASAIO, 36:M372-M375, Li, et al., 1990, Trans. ASAIO, 36:M382-386).
The power obtained from using the muscle in any of the wrap-around configurations, either for blood pump actuation or for direct wrapping around the heart is greatly inefficient, and perhaps physiologically damaging to the muscle. Skeletal muscle is accustomed to pulling in direct tension when it contracts, and the force is dependent on the preload stretch. When the latissimus dorsi is wrapped around the heart or around a blood pump, there is an inefficient pre-stretch of the muscle, and there is also nothing for the muscle to pull against. Muscle capillary blood flow is also impaired during contraction, and mechanisms might be needed to limit preload impairment of muscle blood flow, such as with a solenoid valve which limits the time of filing (Geddes, et al., PACE, 13:783-795, 1990). The net effect with any of these approaches is very poor pumping mechanics with barely sufficient power to augment the circulation. Others have attempted to leave the latissimus dorsi in situ, and have it squeeze a pouch between the muscle and the rib cage (U.S. Pat. Nos. 4,453,537 and 4,771,765), but this has also proven to be an inefficient method of pumping (Chiu, et al., 1987, J. Thoracic Cardiovasc. Surg., 94:694-701, Novoa, et al., 1989, Trans. ASAIO, 35:408-411).
A strong need therefore exists for a system which can maximally utilize skeletal muscle power for a full range of circulatory support needs.